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Wastewater treatment in constructed wetlands: Effects of vegetation, hydraulics and data analysis methods
Linköping University, Department of Physics, Chemistry and Biology, Biology. Linköping University, The Institute of Technology.
2013 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Degradation of water resources has become one of the most pressing global concerns currently facing mankind. Constructed Wetlands (CWs) represent a concept to combat deterioration of water resources by acting as buffers between wastewater and receiving water bodies. Still, constructing wetlands for the sole purpose of wastewater treatment is a challenging task. To contribute to this research area, the fundamental question raised in this doctorate thesis was: how do factors such as vegetation and residing water movements (hydraulics) influence wastewater treatment in CWs? Also, effects of different data analysis methods for results of CW hydraulics and wastewater treatment were investigated. Research was focused on  phosphorus (P), ammonium-nitrogen (NH4+-N) and solids (TSS) in wastewater and o n P in macrophyte biomass. Studies were performed in pilot-scale free water surface (FWS) CW systems in Kenya (Chemelil) and Sweden (Halmstad) and as computer simulations.

Results from the Chemelil CWs demonstrated that meeting effluent concentration standards simultaneously for all water quality parameters in one CW was difficult. Vegetation harvest, and thus nutrient uptake by young growing macrophytes, was important for maintaining low effluents of NH4+-N and P, especially during dry seasons. On the other hand, mature and dense vegetation growing for at least 4 months secured meeting TSS standards. Phosphorus in above-ground green biomass accounted for almost 1/3 of the total P mass removal, demonstrating high potential for P removal through macrophyte harvest in CWs. Also, results suggested that harvest should be species-specific to achieve high P removal by macrophytes and overall acceptable wastewater treatment in CWs. Still, different methods to estimate evapotranspiration (ET) from the Chemelil CWs showed that water balance calculations greatly impacted estimations of wastewater treatment results.

Hydraulic tracer studies performed in the Chemelil and Halmstad CWs showed that mature and dense emergent vegetation in CWs could reduce effective treatment volumes (e-values), which emphasized the importance of regulating this type of vegetation. Also, it was shown that hydraulic tracer studies with lithium chloride performed in CWs with dense emergent vegetation had problems with low tracer recoveries. This problem could be reduced by promoting the distribution of incoming tracer solution into the CW using a barrier near the CW inlet pipe. Computer simulation results showed that the choice of tracer data analysis method greatly influenced quantifications of CW hydraulics and pollutant removal. The e-value could be 50% higher and the pollutant removal 13% higher depending upon used method. Moreover, unrealistic evalues (above 100%) in published literature could to some extent be explained by tracer data analysis method. Hence, to obtain more reliable hydraulic data and wastewater treatment results from CWs, more attention should be paid to the choice of tracer data analysis method.

Abstract [sv]

Konstruerade våtmarker representerar ett koncept för möjligheten att nå en hållbar vattenresurshantering genom att agera som ”filter” mellan föroreningskälla och viktiga vattenresurser såsom sjöar och hav. Mycket kunskap saknas däremot om hur man konstruerar våtmarker med en optimal och pålitlig vattenreningskapacitet. Den här avhandlingen undersöker därför hur vegetation och vattnets väg genom våtmarken (hydrauliken) påverkar avloppsvattenrening i våtmarker. Dessutom undersöktes hur valet av dataanalysmetod av insamlad data påverkar resultaten. Studier genomfördes i Kenya och Sverige i experimentvåtmarker (ca. 40-60 m2) och inkluderadedatainsamling av vattenkvalité, hydraulik (spårämnesexperiment) samt biomassa och fosfor i biomassan av två olika våtmarksväxter. Dessutom genomfördes datorsimuleringar.

Resultaten från Kenya visade att växtskörd och efterföljande näringsupptag av nyskördade växter var viktig för att uppnå låga utgående koncentrationer av fosfor och ammonium i en tropisk våtmark, speciellt under torrsäsongen. Däremot var en välutvecklad och tät vegetation viktig för reningen av partiklar. Fosfor i grön växtbiomassa representerade cirka 1/3 av våtmarkernas totala fosforrening, vilket påvisade potentialen i att genom skörd ta bort fosfor från avloppsvatten m.h.a. konstruerade våtmarker. Resultaten pekade också på att skörden bör vara art-specifik för att uppnå en hög fosforrening och generellt bra vattenreningsresultat. Dock visade olika beräkningsmetoder att vattenbalansen i en tropisk våtmark markant kan påverka vattenreningsresultaten.

Resultaten från spårämnesexperimenten demonstrerade att den effektiva våtmarksvolymen för vattenrening blev mindre vid hög täthet av övervattensväxter. Detta pekade på att regelbunden växtskörd var viktig för att uppnå god vattenrening i våtmarker. Experiment med spårämnet litium visade att man kan få felaktiga resultat p.g.a. att en del spårämne fasthålls på botten i våtmarken om denna har mycket övervattensväxter. Därför bör spridningen av spårämnet i sådana våtmarker underlättas m.h.a. en spridningsbarriär nära inloppsröret. Simuleringar visade också att valet av dataanalysmetod av spårämnesdata starkt kan påverka resultaten och därmed också vår tolkning av en våtmarks hydraulik och reningskapacitet. Den effektiva volymen kunde vara 50% högre och reningseffekten 13% högre beroende på vilken metod som användes. Likaså kan valet av dataanalysmetod ha bidragit till överskattade och orealistiska effektiva volymer (över 100%) i artiklar publicerade de senaste 25 åren. Genom att fokusera mer på valet av dataanalysmetod och t.ex. jämföra resultaten från två olika metoder kan man minimera risken för bristfälliga resultat och därmed felaktiga slutsatser om en våtmarks vattenreningskapacitet.

Place, publisher, year, edition, pages
Linköping: Linköping University Electronic Press, 2013. , 59 p.
Series
Linköping Studies in Science and Technology. Dissertations, ISSN 0345-7524 ; 1509
Keyword [en]
constructed wetland, free water surface flow, wastewater treatment, Kenya, Sweden, vegetation, harvest, Cyperus papyrus, Echinochloa pyramidalis, mass load, phosphorus, ammonium, suspended solids, pollutant removal, hydraulics, residence time distribution, data analysis methods
Keyword [sv]
Konstgjorda våtmarker, avloppsvatten, vattenrening, fosfor, ammonium, partiklar, Kenya, Sverige, växter, Cyperus papyrus, Echinochloa pyramidalis, skörd, hydraulik, dataanalysmetod
National Category
Natural Sciences
Identifiers
URN: urn:nbn:se:liu:diva-91402ISBN: 978-91-7519-649-7 (print)OAI: oai:DiVA.org:liu-91402DiVA: diva2:617656
Public defence
2013-05-30, Planck, Physics House, Campus Valla, Linköpings universitet, Linköping, 13:00 (English)
Opponent
Supervisors
Available from: 2013-04-24 Created: 2013-04-24 Last updated: 2014-10-08Bibliographically approved
List of papers
1. Impact of loads, season, and plant species on the performance of a tropical constructed wetland polishing effluent from sugar factory stabilization ponds
Open this publication in new window or tab >>Impact of loads, season, and plant species on the performance of a tropical constructed wetland polishing effluent from sugar factory stabilization ponds
2007 (English)In: Ecological Engineering: The Journal of Ecotechnology, ISSN 0925-8574, E-ISSN 1872-6992, Vol. 29, no 1, 66-76 p.Article in journal (Refereed) Published
Abstract [en]

The effects of wastewater loading rates and two macrophyte species on treatment of sugar factory stabilization pond effluent were investigated in a pilot-scale free water surface constructed wetland (FWS CW) system in western Kenya. For 12 months, four CWs were operated at a hydraulic loading rate of 75 mm day−1 and four at 225 mm day−1. Half the CWs were planted with Cyperus papyrus and half with Echinochloa pyramidalis. Water samples were taken at the inlets and outlets and analyzed for TP, TDP, NH4-N, and TSS. Mass removal rates of the selected water quality parameters were compared during three periods designated the short rain (period 1), dry (period 2), and long rain (period 3) seasons. There was a significant linear relationship between the mass removal rate of TP, NH4-N, and TSS and the mass load, and season had a significant effect on the mass removal rate of TSS, NH4-N, and TDP. Mass loading rates for TDP were about 78% of those for TP, whereas TDP comprised 78–99% of TP mass outflow rates, indicating a release of dissolved P within the CWs. The only significant difference between the two macrophyte species was associated with mass removal of NH4-N, with more efficient removal in CWs planted with C. papyrus than those with E. pyramidalis. TP mass removal rates were 50–80% higher when a mean water loss for CWs 6–8 during periods 1 and 2 was assumed to represent evapotranspiration for all CWs in period 3 instead of pan evaporation data. This illustrated the importance of accurate estimations of evapotranspiration for pollutant mass removal rates in CWs in tropical climates.

National Category
Natural Sciences
Identifiers
urn:nbn:se:liu:diva-39668 (URN)10.1016/j.ecoleng.2006.07.015 (DOI)000243600700008 ()50668 (Local ID)50668 (Archive number)50668 (OAI)
Available from: 2009-10-10 Created: 2009-10-10 Last updated: 2017-12-13
2. Free water surface constructed wetlands for polishing sugar factory effluent in western Kenya: macrophyte phosphorus recovery and treatment results
Open this publication in new window or tab >>Free water surface constructed wetlands for polishing sugar factory effluent in western Kenya: macrophyte phosphorus recovery and treatment results
2006 (English)In: Proceedings of the 10th International Conference on Wetland Systems for Water Pollution Control, 23-29 September 2006 / [ed] Dias, V., Vymazal, J., 2006, 709-718 p.Conference paper, Published paper (Refereed)
Abstract [en]

Wastewater treatment and nitrogen and phosphorus (P) recovery in harvested biomass of two macrophyte species receiving two wastewater loading rates was studied in a free water surface constructed wetland (FWS CW) in Kenya. Half the CWs were planted with Cyperus papyrus and half with Echinochloa pyramidalis. Inlets and outlets water samples were analysed for selected water quality parameters. Macrophytes were harvested at around 7 month intervals on three occasions for determination of biomass, P and N content. Area specific removals of TP, TSS and Nh4+-N were higher in the high-load CWs and in the low-load ones, but the relative removal was lower. For Nh4+-N, there was a significantly higher removal in C. papyrus CWs- Each macrophyte species had similar tissue P content independent of mass load suggesting excess available phosphorus in all CWs, as supported by the low N:P ratios. During a 7 month period, the amount of P stored daily in the green biomass of the macrophytes represented 18-29% and 25-100% of the daily removal of TP and TDP, respectively.

Keyword
Free water surface wetland; Phosphorus removal; N uptake; P uptake; Cyperus papyrus; Echinochloa pyramidalis
National Category
Natural Sciences
Identifiers
urn:nbn:se:liu:diva-91400 (URN)
Conference
The 10th International Conference on Wetland Systems for Water Pollution Control, 23-29 September 2006; Ministério de Ambiente, do Ordenamento do Territóri e do Desenvolvimento Regional (MAOTDR) and IWA: Lisbon, Portugal
Available from: 2013-04-24 Created: 2013-04-24 Last updated: 2013-04-24Bibliographically approved
3. Hydraulic performance of small free water surface constructed wetlands treating sugar factory effluent in western Kenya
Open this publication in new window or tab >>Hydraulic performance of small free water surface constructed wetlands treating sugar factory effluent in western Kenya
2012 (English)In: HYDROLOGY RESEARCH, ISSN 1998-9563, Vol. 43, no 4, 476-488 p.Article in journal (Refereed) Published
Abstract [en]

This study investigated, using a lithium salt tracer, how macrophyte species and hydraulic loading rate (HLR) of wastewater influenced hydraulics in constructed wetlands (CWs). Four pilot-scale CWs received 45 mm day(-1) of pre-treated sugar factory effluent and another four received 110 mm day(-1). Half the CWs were planted with Cyperus papyrus and half with Echinochloa pyramidalis. Results showed a significant negative connection between tracer mass recovery and wetland water leakages. Also, a significant negative relationship between active wetland water volume and macrophyte density was detected. Further, a significant effect of HLR on mass removal rates of NH4+-N was observed. However, no significant effect of either HLR or macrophyte species on wetland hydraulic parameters was found.

Place, publisher, year, edition, pages
IWA PUBLISHING, 2012
Keyword
constructed wetland; Cyperus papyrus; Echinochloa pyramidalis; hydraulic loading rate; hydraulic tracer study; vegetation
National Category
Engineering and Technology
Identifiers
urn:nbn:se:liu:diva-79711 (URN)10.2166/nh.2012.133 (DOI)000306235400015 ()
Available from: 2012-08-13 Created: 2012-08-13 Last updated: 2013-04-24
4. Tracer behaviour and analysis of hydraulics in experimental free watersurface wetlands
Open this publication in new window or tab >>Tracer behaviour and analysis of hydraulics in experimental free watersurface wetlands
Show others...
2012 (English)In: Ecological Engineering: The Journal of Ecotechnology, ISSN 0925-8574, E-ISSN 1872-6992, Vol. 49, 201-211 p.Article in journal (Refereed) Published
Abstract [en]

Effects of inlet design and vegetation type on tracer dynamics and hydraulic performance were investigated using lithium chloride in 18 experimental free water surface wetlands. The wetlands received similar water flow but had different vegetation types: 6 emergent vegetation wetlands (EVWs), 6 submerged vegetation wetlands (SVWs) and 6 free development wetlands (FDWs). Two types of inlet designs were applied: half of each wetland vegetation type had a barrier near the inlet to help distribute incoming tracer solution, while the rest had no barrier. Residence time distribution (RTD) functions were calculated from tracer data using two techniques: method of moments and a novel Gauss modelling approach. RTD functions were used to quantify hydraulic parameters: active wetland volume (e-value), water dispersion (N-value) and hydraulic efficiency (λ-value).

For wetlands without barrier, significantly lower tracer mass recoveries were found from EVWs compared to FDWs and SVWs, signifying a risk of tracer methodological problems in small densely vegetated wetlands. These problems were minimized in wetlands with an inflow construction promoting distribution of incoming tracer solution.

Compared to the method of moments, Gauss modelling seemed to produce more reliable λ-values but less reliable N-values. Data for precise hydraulic quantification were lost by Gauss modelling, as indicated by overall lower variance in these data sets and lower mass recoveries. However, Gauss modelling may minimize uncertainties associated with lithium immobilization/mobilization. Parameters were significantly affected by the RTD data analysis method, showing that the choice of method could affect evaluation of wetland hydraulics.

The experimental wetlands in this study exhibited relatively high e-values and low N-values. This was probably caused by the small size of the wetlands and low water flow velocities, emphasizing that hydraulic parameter values obtained in small experimental wetlands may not be applicable to hydraulics in larger wetlands.

The method of moments revealed lower e-values from EVWs compared to SVWs and FDWs. It was indicated that lower e-values were mainly caused by vegetation volumes. This highlighted a need for regular maintenance to secure efficient treatment volume in wetlands with dense vegetation.

Keyword
Lithium tracer, Hydraulic performance, Free water surface wetlands, Vegetation, Inlet, Gauss modelling, Data analysis
National Category
Natural Sciences
Identifiers
urn:nbn:se:liu:diva-86981 (URN)10.1016/j.ecoleng.2012.07.009 (DOI)000320931100029 ()
Available from: 2013-01-08 Created: 2013-01-08 Last updated: 2017-12-06
5. Diluting the evidence? How residence time analyses can influence your results
Open this publication in new window or tab >>Diluting the evidence? How residence time analyses can influence your results
(English)Manuscript (preprint) (Other academic)
Abstract [en]

Hydraulic tracer studies are frequently used to estimate wetland residence time distributions (RTDs) and ultimately pollutant removal. However, there is no consensus on how to analyse these data. We set out to (i) review the different methods used and (ii) use simulations to explore how the data analysis method influences the quantification of wetland hydraulics and pollutant removal. The results showed that the method influences the water dispersion (N) most strongly and the removal least strongly. The influence increased with decreasing effective volume ratio (e) and N, indicating a greater effect of the method in wetlands with low effective volume and high dispersion. The method of moments with RTD truncation at 3 times the theoretical residence time (tn) and tracer background concentration  produced the most dissimilar parameters. The most similar parameters values were those for gamma modelling and the method of moments with RTD truncation at tracer background concentration. For correct removal estimates, e was more important than N. However, the results from the literature review and simulations indicated that previously published articles may contain overestimated e and underestimated N values as a result of frequent RTD truncations at 3tn when using the method of moments. As a result, the removal rates may also be overestimated by as much as 14% compared to other truncation methods or modelling. Thus, it is recommended that wetland hydraulic tracer studies should use the same method, specifically, RTD truncation. We conclude that the choice of tracer data analysis method can greatly influence the quantifications of wetland hydraulics and removal rate.

Keyword
Gamma model, removal rate, residence time analysis, tracer, trapezoid rule, wetland hydraulics
National Category
Natural Sciences
Identifiers
urn:nbn:se:liu:diva-91401 (URN)
Available from: 2013-04-24 Created: 2013-04-24 Last updated: 2014-10-08

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